Number of sources is not strict as long as it is sufficient for a high-quality essay (can be electronic or books – no limit to the number of internet sources). It is essential to also consider conflicting evidence and explain why it does not undermine key arguments made (counter arguments). Moreover, the introduction of the essay should also mention the other factors (apart from those discussed in the essay) that do affect the make or buy decisions (showing the awareness that other factors exist and may provide reasons why they are not discussed in the essay, perhaps the ones discussed are more important). Limitations of the essay must also be discussed. Lastly, I think that ‘asset specificity’ is one of the factors that should be discussed as it has been emphasised by many research.
